A radio frequency (RF) signal usually consists of a carrier with information modulated on it. The task of the front-end of a radio receiver is to amplify the RF signal and mix it either to base band or to some intermediate frequency (IF) such that the information can be conveniently detected. Reception of a desired RF signal is often depreciated by the presence of an interference signal being received along with the RF signal. When the interference signal is of a constant frequency, it is usual practice to substantially eliminate the effect of the interference signal by using a filter for that purpose, in the radio receiver.
A radio receiver in which the signal is converted directly to base band is known as a homodyne or direct conversion receiver. A radio receiver in which the signal is converted to some IF is referred to as a superheterodyne receiver. The direct conversion receiver is simpler then the a superheterodyne receiver, and thus would be preferred for integrated circuit manufacture were it not for various inherent problems. One of these problems is that of local oscillator (LO) signal self mixing. Another of these problems is that of direct current (DC) voltage offsets which accumulate due to the some of the received information being close to the carrier frequency. Thus the greater complexity and cost of superheterodyne receivers is generally accepted as necessary for satisfactory communications, however superheterodyne receivers also have various inherent problems.
In a superheterodyne radio receiver one of the inherent problems is the generation of an image frequency signal. For example, an RF input signal is amplified in a low noise amplifier (LNA) before being mixed with some reference frequency signal generated by a local oscillator. The mixing produces a signal at a frequency being a difference of the reference frequency signal and the RF input signal. This difference frequency signal is referred to as an intermediate frequency (IF) signal. The IF signal is passed through a band pass amplifier, sometimes referred to as an IF strip or an IF stage, to eliminate unwanted signals and noise before detection of the information. One of the inherent problems in a superheterodyne receiver is that the mixing process generates an image frequency signal which is removed from the RF input signal by a sum of twice the difference between the reference frequency and the RF input signal. Typically the power of the image frequency signal is great enough that if only a small fraction of this signal leeks to the input of the superheterodyne receiver, it is also amplified along with the desired RF input signal. If any of the image frequency signal reaches the input of the mixer it is mixed down into the IF along with the input signal and interferes with the detection of the information being received in the RF input signal. Hence before the unwanted image frequency signal reaches the mixer it is desirably removed. Typically this is done with a filter that attenuates the image frequency signal.
In many radio receivers of present day manufacture, off-chip passive filters, such as surface acoustic wave (SAW) filters or ceramic filters, are used for image rejection. Since neither SAW nor ceramic filters have been practically implemented in an integrated circuit form, these filters represent a major impediment to reducing the cost of integrated circuit radio receiver a apparatus. The use of external filters also complicates the design of the radio receiver front-end. The output of a LNA drives a load impedance of the off-chip filter, typically of 50 ohms impedance, via two bond wires in the package used for the receiver. Bond wires are a major source of signal radiation. The bond wires contribute significantly to detrimental signal coupling or leakage between ports of the receiver, including the input port where the radio frequency signal is being received. Additional pins to accommodate the bond wire connections with the filter also increase the cost of packaging. This additional cost along with the cost of SAW or ceramic filters is a significant portion of the overall cost of a typical integrated circuit radio receiver.
Several filter types have been integrated in on-chip implementations of image rejection filters but in operation exhibit comparatively less desirable performance characteristics than the typical off-chip implementations of image rejection filters. Some designs have employed an additional stage to the radio receiver following the LNA for filtering out the image frequency signal. Implementations of these designs characteristically require significant extra circuitry which occupies corresponding significant extra chip area. In operation these designs consume a significant extra operating current. This extra circuitry is in series with the signal path of the received RF signal. Examples of these receivers typically exhibit poor noise characteristics and less than desirable linearity. On the other hand, advantageously the signal never has to leave the chip before reaching the IF stage in the receiver chain. If the reduction in the quality of detected information is tolerable, a more economical integrated circuit package is used as the beam leads and package pin-out needed for off-chip filter designs are avoided.
It has been suggested that a filter can also be included within a conventional integrated LNA topology to reduce amplification of an image frequency signal. A conventional LNA includes two transistors connected in cascode. In one example energizing power is applied to an output collector electrode of one transistor through a parallel resonated circuit having a low direct current resistance and a higher impedance at the frequency of signals intended for amplification. An emitter electrode of the other transistor is coupled to via an inductor to ground. Bias is applied to hold the transistors in a linear operating range such that RF signals coupled to the other transistor appear amplified at the output collector electrode. The inductor is constructed to have a value which presents a higher impedance at the emitter electrode for the unwanted image frequency signal than for signal frequencies of desired RF signals. Below the resonant frequency of the inductor the LNA operates with an inductive degeneration characteristic. It has also been suggested that an active tunable capacitance in parallel be used with the inductor to lower impedance and consequently increase gain at the desired RF signal frequencies.
In spite of these suggested integrated designs, at present if a radio receiver of uncompromised image rejection and low noise performance is required, the radio receiver is expected to include an external filter or filters of the SAW or ceramic filter types.
Consequently it is an object of the invention to reduce interference normally associated with image frequency signals in integrated circuit radio receivers.